This invention relates generally to seals or gaskets, and more particularly to a spirally wound gasket.
Spirally wound gaskets known in the art generally consist of alternate layers of a thin gauge metal strip, such as stainless steel, and a thin strip of a soft, non-metallic filler material which together are wound in a spiral to form an annular ring, as is a roll of adhesive tape. The layered edges of the strips form the sealing surfaces. Suitable filler materials include various fluorocarbon polymers, asbestos, Grafoil.RTM., and ceramic.
The metallic strip is typically formed with a central, generally V-shaped fold or indentation throughout its length, and to either side of the fold are substantially flat portions. The filler material assumes this shape also.
The purpose of the V-shaped fold is to establish a path along which the metallic strip can deform in a resilient or elastic manner, as the gasket is compressed between the surfaces to be sealed. Effectively, the apex of the V-shape acts as a hinge about which the legs of the V pivot. Since the filler material itself is usually soft and non-resilient, the metallic strip is required to force the filler to interfere with the surfaces to be sealed.
One drawback of the previous spiral wound gaskets is that they tend to yield at high temperatures, typically, above 700.degree. F. Due to the high degree of compression which the gaskets are subjected to, the yielding results in permanent or plastic deformation, and a loss of resiliency. This yielding occurs particularly when temperatures repeatedly cycle from high to low, and the resulting lack of resiliency causes a relatively high degree of leakage through the gasket.
Due to this leakage, spiral wound gaskets are usually not acceptable for use in "fire-safe" valves. To qualify as "fire-safe", a valve is subjected to high temperature in an open flame environment. Over a fixed period of time during the test, leakage from the valve must not exceed a certain, limited quantity. A major application for fire-safe valves is in the transport of inflammable fluids, the leakage of which during a fire can be disastrous.
The sealing characteristics of prior spiral wound gaskets are also unsatisfactory in cryogenic applications, where fluids are below -100.degree. F. The cause of decreased sealing capability at lower temperatures is also a loss of resiliency, in this case due to the contraction and brittleness of the metal strip.
Another disadvantage of these gaskets is that the bolt torque on the joint being sealed must be very high in order to sufficiently compress the gasket to achieve a tight seal. As a result, installation of the gaskets is lengthy, and the gaskets are not suitable for use in certain applications where the bolts cannot withstand the required torque.
Thus, a need exists for a resilient, low-torque loaded gasket which is effective under severe temperature conditions.